1. Field of the Invention
The present invention relates generally to image scanner technology and, more specifically, to a system and method for improving the scanning speed while maintaining the output resolution of the image scanner.
2. Discussion of Related Art
Digitizing image scanners are often used to digitize documents so they can be manipulated using a computer. For example, a scanner can be used to digitize text and put it into a format recognized by a word processing program so that an editor can rewrite or modify the document.
With advances in processor speed and scanner technology, it has become advantageous to also digitize pictorial images as well as printed text. In an effort to enhance scanner performance, developers have sought ways to optimize the number of pixels used to represent a digital image. A simple reduction in the number of pixels used to represent an image results in a reduction in image resolution. An increase in the number of pixels results in greater processor time required to digitize the image and perform subsequent image processing.
One of the most useful integrated devices is the charge-coupled device (CCD). The CCD is part of a broader class of structures known generally as charge transfer devices. These are dynamic devices which move charge along a predetermined path under the control of clock pulses. These devices find applications in memories, various logical functions, signal processing, and in imaging (e.g., image scanners).
The CCD stores a charge referred to herein as a pixel charge. The pixel charge stored in the CCD 120 is produced by a light sensitive diode (not shown). The amount of charge the light sensitive diode stores into the CCD is a function of how much light is hitting the light sensitive diode and the time of exposure to the light. Cutting the time of exposure in half results in half the charge being stored in the CCD. The (analog) pixel charge stored in the CCD is shifted out of the CCD and digitized into a digital value.
Scanning speed is often paced by how fast pixels in an image can be converted into digital form. When scanning at lower than maximum resolution, traditionally all of the raw pixel elements of a CCD are processed even if only every other pixel will be used. When scanning at a lower resolution, particularly a resolution that is less than half of the native resolution of the scanner, it is inefficient to have to process every pixel when many of the processed values will be discarded. If the shift rate of the CCD is doubled and only every other pixel charge is sampled (assuming a resolution of one half is desired), the time to digitize the pixel charges would remain constant while the time to shift out a line of pixel charges is reduced by a factor of two.
The drawback to this method is that the exposure time (and hence signal level) is equal to the time it takes to shift out a line of pixel charges from the CCD when scanning as fast as possible. This means that the signal to be digitized will be half the level it was at the slower speed and with constant system noise, the signal to noise level will be degraded by a factor of two.
As such, it would be desirable to have a procedure for increasing the scanning speed of an image scanner while still maintaining image quality of the image when less than full resolution is needed.